The present invention relates to a cylinder head cover attached to the cylinder head of an engine, and more particularly, to a cylinder head cover that includes a synthetic resin outer shell and a valve case of an oil control valve, which valve case is firmly fixed to the outer shell by being insert-molded in the outer shell.
Conventionally, a cylinder head cover attached to the cylinder head of an engine has an outer shell, and the outer shell is made of synthetic resin to reduce the weight. A valve case for attaching an oil control valve is installed in the synthetic resin outer shell. Alternatively, the outer shell is molded with the valve case inserted in a mold. A cylinder head cover having a valve case and the outer shell fixed to and integrated with each other as described above has been developed.
Japanese Laid-Open Patent Publication No. 2007-100657 (a first prior art) discloses an attaching structure that facilitates the attachment of a valve case to an outer shell. According to this structure, a valve case is fixed to a cam cap by a bolt passed through a boss of the valve case after the valve case is temporarily joined to an outer shell. Simultaneously, using the bolt, the outer shell and the valve case are secured to each other through a cylindrical body provided on the outer surface of the boss. Also, an O ring is provided between the cylindrical body and the boss, so that oil in the cylinder head cover does not leak to the outside by trickling along the inner surface of the cylindrical body. The O-ring is pressed when the cylindrical body is welded to the outer shell, and tightly held between the lower end of the cylindrical body and the upper surface of the valve case.
Japanese Laid-Open Patent Publication No. 2007-107479 (second prior art) discloses an attaching structure in which a groove is provided between an outer shell and a valve case. Adhesive is provided in the groove to prevent oil from leaking. That is, in this attaching structure, a synthetic resin outer shell 3 and a valve case 2, which form a cylinder head cover 1, are integrally formed by insert molding. The valve case 2 has a bolt fastening portion 5, and the valve case 2 is fixed to a cam cap 4 together with the outer shell 3 by means of a bolt 6. An annular groove 40 is formed in the upper surface of a flange 41 provided on the outer circumference of the bolt fastening portion 5. Adhesive is applied to the groove 40 prior to the molding process of the cylinder head cover 1 in which the valve case 2 is inserted. The adhesive is prevented from being washed away by injected synthetic resin, and remains in the groove 40. Accordingly, the cured synthetic resin and the adhesive of the valve case 2 are jointed to each other. Therefore, in the vicinity of the bolt fastening portion 5, oil is prevented from leaking out of the valve case 2 through the joint portion between the valve case 2 and the outer shell 3.
The attaching structure of the first prior art allows the valve case to be temporarily joined to the outer shell, and therefore facilitates the assembly. However, the first prior art involves burdensome steps, in which the cylindrical body and the outer shell need to be welded to each other, while keeping the O-ring pressed by the end face of the cylindrical body.
In the second prior art, the outer circumferential surface 42 of the flange 41 closely contacts a contact surface 43 of the outer shell 3 after injecting synthetic resin. However, due to the molding shrinkage at the solidification of the synthetic resin, the outer circumferential surface 42 of the flange 41 can slightly separates from the contact surface 43 of the outer shell 3. Repetitive changes between high-temperature environment of the running engine and thermoneutral environment of the stopped engine cause the separation of the contact surface 43 from the outer circumferential surface 42 to advance upward from the lower end of a joint portion 44. This repeatedly applies stress on the adhesive in the groove 40, which eventually destroys the adhesive. As a result, oil can leak to the outside of the valve case 2 through destroyed portion of the adhesive.